In a conventional mobile communication system, when a communication link is not established between a mobile station UE and a radio base station NodeB, the mobile station UE needs to transmit a signal to the radio base station NodeB by some means.
The mobile station UE can determine a transmission power to be used at the mobile station UE, by using a random access channel (hereinafter, RACH) at a physical layer.
As an example of transmitting a signal to the radio base station NodeB through the RACH, a method using a power ramping can be considered. In this method, the mobile station UE transmits a single random access request as needed, and recognizes a failure of the transmission when an acknowledgment of the random access request transmitted from the radio base station NodeB cannot be received.
An example of the power ramping in the W-CDMA (Wideband Code Division Multiple Access) will be described. Firstly, the mobile station UE transmits, to the radio base station NodeB, a preamble signal as the random access request. Here, the mobile station UE transmits the random access request while gradually increasing the transmission power.
The radio base station NodeB receives the random access request from the mobile station UE, and transmits a random access response for notifying the receipt of the random access request, to the mobile station UE through a downlink control channel.
In response to the receipt of the random access response from the radio base station NodeB, the mobile station UE stores the transmission power used for the most recent transmission of the random access request, which is transmitted to the radio base station NodeB.
Subsequent to the random access request, the mobile station UE transmits, to the radio base station NodeB, an allocation request for a control channel or the like, by using a transmission power equal to or higher than the stored transmission power.
The radio base station NodeB receives the allocation request from the mobile station UE, and transmits an allocation of a control channel or the like, to the mobile station UE. Thereby, the communication link is established between the mobile station UE and the radio base station NodeB (see Non-patent document 1 and Non-patent document 2, for example).
However, in the conventional mobile communication system, a collision can be occurred when multiple mobile stations UE simultaneously transmit signals to the radio base station NodeB by using the RACH (Random Access Channel). In that case, the conventional mobile communication system has a problem that the mobile station UE erroneously determines that the radio base station NodeB has received the random access request transmitted from the mobile station UE.
With reference to FIG. 11, an example in which the mobile station UE makes an erroneous determination will be described. In FIG. 11, neither a mobile station UE #1 nor a mobile station UE #2 has a communication link with a radio base station NodeB. Here, an example will be described for a case in which the mobile station UE #1 erroneously determines that an access request has been successfully transmitted to the radio base station NodeB although the transmission of the access request has not been successful.
In Step S2001, the mobile station UE #1 transmits a random access request to the radio base station NodeB through the RACH.
Here, assume that the base station NodeB has not received the random access request transmitted from the mobile station UE #1 for a reason that the transmission power is too small, or the like.
In addition, in Step S2001, the mobile station UE #2 transmits a random access request to the radio base station NodeB through the RACH at the same timing as the transmission of the random access request from the mobile station UE #1.
Here, each of the random access request transmitted from the mobile station UE #1 and the mobile station UE #2 collide with each other, and is transmitted to the radio base station NodeB.
In Step S2002, the radio base station NodeB receives the random access request, and transmits a random access response for notifying the receipt of the random access request, to the mobile station UE #1 through a downlink shared control channel corresponding to the RACH used for transmitting the random access request.
In addition, in Step S2002, the radio base station NodeB receives the random access request, and transmits a random access response for notifying the receipt of the random access request, to the mobile station UE #2 through the downlink shared control channel corresponding to the RACH used for transmitting the random access request.
In Step S2003, the mobile station UE #1 receives the random access response through the downlink shared control channel, and then transmits an allocation request to the radio base station NodeB.
Here, the mobile station UE #1 erroneously determines that the random access request transmitted from the mobile station UE #1 has been received at the radio base station NodeB. Moreover, the transmission power used for transmitting the allocation request from the mobile station UE #1 to the radio base station NodeB does not allow the allocation request to be received at the radio base station NodeB.
In Step 2004, the mobile station UE #2 receives the random access response through the downlink control channel, and then transmits an allocation request to the radio base station NodeB.
In Step S2005, the radio base station NodeB transmits an allocation response for notifying the receipt of the allocation request, to the mobile station UE #2 through the downlink control channel.
As described above, the conventional mobile communication system has a problem that the mobile station UE may erroneously determine that the random access request transmitted from the mobile station UE has been received at the radio base station NodeB.
Further, in that case, another problem arises that a processing delay occurs at the mobile station UE until the determination made by the mobile station UE is turned out to be erroneous.
Moreover, in that case, still another problem arises that resource is wasted at the mobile station UE until the determination made by the mobile station UE turns out to be erroneous.    Non-patent document 1: 3GPP TS 25.213 V5.5.0    Non-patent document 2: 3GPP TS 25.211 V5.6.0